This invention relates to a speed governor for adjusting the speed of an input rotation by causing a braking force to act upon the input rotation by eddy current of an electroconductive plate.
An apparatus disclosed in Japanese Utility Model Unexamined Publication No. 19808/1986 has a construction wherein a magnetic circuit is constituted by a copper disk mounted on the final stage of a speed increasing wheel train, and lo multipolarized permanent magnets are arranged so as to have the copper disk therebetween. A magnetic force is applied to the copper disk during its rotation to cause a braking force to be generated by eddy current of the copper disk.
The braking force is changed by rotating the permanent magnets radially and thereby varying the magnetic flux density between the upper and lower permanent magnets.
In the above construction, two or more magnets are necessary, which is costly. In addition, the braking force of the permanent magnets comes in two levels, which, due to variations in the magnetization of the permanent magnets and in other mechanical parts, causes large variations in its performance. Further, the two-level adjustment does not permit fine adjustment, limiting the scope of application of the device.
Moreover, the attracting force of each permanent magnet is strong for the small size of the case, and when the permanent magnets are rotated in the radial direction, it is difficult to maintain a coercive force for locking excessive rotation after their rotation.